Sinus tachycardia in the early hours of STEMI is a very common arrhythmia. This seemingly simple problem can be really worrisome to many cardiologists and give sleepless nights(While the patient may sleep comfortably !)
The importance of sinus tachycardia in STEMI primarily lies in answering the following question
A. Is it compensatory sinus tachycardia ? , Where in , the left ventricle is struggling to maintain the cardiac output and works more per minute to maintain the vital cardiac index
Or
B. Is it a non compensatory -Inappropriate Sinus tachycardia ? It is a simple response to heightened adrenergic tone and increased neural traffic from the injured ventricular myocardium . (or high baseline anxiety levels )
It should be recalled , tachycardia in any form is detrimental following STEMI as it increases the MVo2 ie myocardial oxygen consumption. This is the reason , beta blockers are administered in this situation. Compensatory tachycardia denotes , myocardium is working at it’s reserve capacity , to prevent an LVF that is impending . Hence , one should recognise , the compensatory tachycardia can not be tampered with , as we like ! .
How do you clinically differentiate a largely benign inappropriate tachycardia from potentially harmful compensatory tachycardia ?
It is not an easy task. Heart rate is typically around 120/mt in compensation to impending LVF. While inappropriate tachycardia has no limits , it can exceed up to 140 or so . Further , tachycardia due to LV dysfunction has reduced variability.( Typically hovers around 120( +/- 5) .In stress or anxiety heart rate fluctuates more .
Accompanying S3 suggest compensatory tachycardia . Even a few basal crackles would make a diagnosis of LVF .
The definite way to differentiate could be ( Also the dangerous way ! ) looking for therapeutic worsening to beta blockers .
How to control the sinus tachycardia in STEMI ?
Beta blockers are the mainstay. Any of the beta blockers , metoprolol, atenolol, carvidilol can be used.Oral metoprolol up to 50 mg can be used. Beta blocker usage is primarily useful in non compensatory tacycardia. It should be realised , the wide spread routine use of intravenous beta blockade has largely been discontinued as it has adverse outcome.The greatness of carvidilol, in cardiac failure mainly applies to stable chronic cardiac failures. So , it is important to recognise, carvidilol can not be used liberally , in sinus tachycardia associated with impending or manifest LVF in STEMI.
*The potential source for tachycardia like dopamine, dobutamine etc should be excluded.
Other options are
Digoxin ( Not withstanding the critics , it is still useful in acute MI with persistent sinus tachycardia ,The advantage is , it can be used without a need to differentiate whether it is compensatory or non compensatory!)
Ivabradine , a wonder drug supposed to reduce selcetively reduce the sinus rate without negative inotropic action could be tried.(Data lacking for this use )
Final message
Sinus tachycardia , may be seen as a simple arrhythmia . but, the circumstances in which it occurs , it’s mechanism and the limited therapeutic options , narrow safety margin of beta blockers , makes it a interesting clinical issue.
Dr.S.Venkatesan MD 
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